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Although the U.S. Food and Drug Administration generally considers 400-milligram doses of caffeine to be safe, energy drinks can pose greater risks because of their proprietary ingredient blends, which often include the acid compound taurine, according to the study.

For the controlled study, researchers from the David Grant U.S.A.F. Medical Center at Travis Air Force Base in Fairfield, California, divided 18 participants into two groups. One group consumed a commercial energy drink that included 108 grams of sugar and 320 milligrams of caffeine. The second group consumed a control drink containing 320 milligrams of caffeine, 40 milliliters of lime juice and 140 milliliters of cherry syrup in carbonated water.

The researchers measured participants' peripheral blood pressures (outlying arteries) and central blood pressures (near the heart) at two-hour intervals. Two hours in, they found that those in the energy drink group had a corrected QT interval 10 milliseconds higher than the control group. Medications that affect the corrected QT interval by 6 milliseconds must carry a warning label on them, but energy drinks do not have the same requirement.

“The QT interval is the measurement of the time it takes ventricles in the heart (the lower chambers) to repolarize, or prepare to generate a beat again,” lead researcher Emily A. Fletcher said in the report. “It’s the pause from the end of the electrical impulse generating the heart to beat to the next impulse.”

Fletcher noted that commercial medications that affect this interval by up to 6 milliseconds are sold with label warnings. If this interval is too short or too long, it can result in life-threatening arrhythmia.

The researchers also found that both groups experienced increases in systolic blood pressure, but levels in the control group returned to normal after six hours. Further research is required to determine the long-term effects on the energy drink group, the researchers said in their report.

The researchers discouraged energy drink consumption from those with high blood pressure or heart conditions.